Hydrostatically powered driven equipment such as lawn tractors have become extremely popular and many utilise the axial piston swash-plate configuration for both the pump and motor elements of the hydrostatic transmission. Such tractors generally have an internal combustion engine having a vertical crankshaft which is connected to the transaxle by means of a conventional belt and pulley arrangement. A standard hydrostatic transmission for such a transaxle includes a hydraulic pump, which is driven by an input shaft from the engine output by means of the belt and pulley arrangement, and a hydraulic motor, both pump and motor are mounted on a center section located inside the transaxle housing. Rotation of the pump by an input shaft creates axial motion of the pump pistons during periods when the pistons are operating against an inclined thrust or swash-plate. The fluid flow thus created by the reciprocating axial motion of the pistons is channelled via porting and passages in the center section to the hydraulic motor, with the effect that the incoming fluid causes the pistons of the motor to reciprocate and create a turning moment that causes rotation of the hydraulic motor. The hydraulic motor in turn has an output shaft which drives the vehicle's axles through speed-reducing gears and a mechanical differential. Examples of such hydrostatic transaxles are shown in the following patents: U.S. Pat. Nos. 5,090,949; 5,473,964 and 5,501,640.
All three references use an axial piston swash-plate pump and motor respectively engaged to a center section which is located within a two-shell housing structure. The main purpose of the center section is to provide a fluid link between the pump and the motor and allowing the transmission of hydraulic power. Patents '964 and '640 in the names of Okada and Hauser respectively, teach the use of an input shaft driven pump where the swash-plate lies adjacent to the upper housing. This contrasts with the disclosure in patent '949 which teaches the use of bevel gears for connecting the input-shaft to the pump and-wherein this example of prior art, the swash-plate of the pump lies directly across both the upper and lower housings of the transaxle.
The center section shown in all three above references require numerous machining operations to prepare the initial raw casting to be ready for use. For instance: drilling some or all of the internal flow passages and arranging retaining means so that plugs/valves and such like can be subsequently fitted to close off the flow circuit; making good two of the faces which provide the fluid coupling means for the pump and motor, and when required, for the subsequent attachment of the valve-plates; preparation of mounting surfaces for attaching the center section to the housing structure. Furthermore, the upper transaxle housing aluminium diecasting itself requires a number of machining operations before it can be used such as the provision for the shaft bearing and seal as well as hole or holes and seals for the control lever and various associated linkages.
As a general rule, the more machining operations required in the upper transaxle housing casting as well as the more complex operations required in the center section casting, the greater the cost of manufacture of the complete hydrostatic transaxle. Therefore the reduction in the number of such machining operations and by grouping them into one rather than two components would save expense.
Although only shown in the '949 patent, almost all hydrostatic transaxles make use of a cooling fan mounted to the input drive shaft in an attempt to help prevent the internal components and fluid from overheating. However, the prior art teaches a center section which although attached in some manner to the interior of the housing, it is still essentially a separate entity from the transaxle housing. As a result, effective cooling of the fluid passing through the passages in the center section that connect the pump and motor together is hindered as the fluid surrounding the center section acts as a insulator to slow down the rate of heat transfer from the power transmission fluid in said passages to the surrounding housing radiator.
The amount of heat able to be radiated away from the transaxle housing exterior to the surrounding environment is of course greatly enhanced over that region on the boundary of the transaxle housing that lies directly in the path of the air flow from the cooling fan. However, it is apparent that although the fluid inside the housing nearest that region where the fan is operating is being cooled, fluid elsewhere may still remain at very high temperature. Perhaps more importantly, as the fluid circulating between the pump and motor in the fluid passages in the center section becomes extremely hot during operation, especially when the unit is heavily loaded and used in a high ambient temperature environment, the resulting drop of operating efficiency due to decreasing fluid viscosity and a corresponding increase in fluid leakage losses can be a concern with the prior art.
This problem exists because the attendant power losses associated with such close coupled pump and motor combinations produce a lot of unwanted heat due to the rapid fluid compression/decompression cycles and general friction between the sliding surfaces. Such losses causes the fluid circulating between the pump and motor through the center section to become extremely hot, and because the prior art teaches a transaxle housing structure whereby the internal fluid reservoir completely or almost completely surrounds and insulates the center section, these prior-solutions are not conducive to the promotion of most effective cooling for the circulating fluid in the centre section flowing in a closed-loop circuit between the pump and motor. This limitation occurs because the bulk of the heat accumulating in the center section can only be transferred by conduction to the surrounding hydraulic fluid and then through the fluid itself to reach the boundary walls of the housing surrounding the fluid chamber from where it can be radiated away to the surroundings. The remove of unwanted heat from the center section consequently takes time.
Therefore in these prior devices where the center section is effectively insulated by the surrounding hydraulic fluid, the delay in the transfer of unwanted heat out of the transaxle may on occasion result in the fluid of the hydrostatic transmission becoming overheated with the risk that the operational life of the fluid is shortened or that the lubricating properties of the fluid deteriorates to the extent that threatens the useful operational life of the hydrostatic transaxle.